You can grab a 100, 000 volt power line and hang from it and you will be fine. So long as you are just hanging from it. Now reach out and touch a grounded item (steel tower, maybe a tree if its wet) and that power will now flow through you and towards the ground, and you won't be quite so fine.

The squirrel is fine so long as its only running along the power line. Its when its transitioning from the power line to something else that its a little risky.